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Localized Attack near Welds: In Fertilizer Plants, Downhole, Gas Plants, Offshore, Downstream, Pipelines … and the List Goes on

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  7. Localized Attack Near Welds

Carbon steel welds and their surrounding regions get picked on. CO2 is a frequent, but not the only culprit in services ranging from downhole conditions through to fertilizer plants. Hydrofluoric, sulfuric and other acids can also result in localized attack.

The carbon steel header shown below had been in service for more than 30 years at approximately 200 °F; it exemplifies a localized attack. As the fertilizer service equipment aged and the process changed, more and more leaks were experienced in the nozzles and areas with turbulence.

The top left image shows one of the line nozzles; its red arrow points at the localized weld attack. The top right image shows a metallographic cross-section of the attacked weld. The bottom left image is a close-up of the scale around the nozzle’s attacked areas. Note that the remnant cementite from the steel’s pearlite is visible within the corrosion product. The bottom right Energy Dispersive X-ray (EDX) image highlights a scale rich in oxygen and carbon.

In this instance, CO2 resulted in the localized carbon steel attack since:

  • Scale within pits contained iron, carbon, and oxygen, as shown in the EDX image. This composition is consistent with siderite (FeCO3). Siderite is a common corrosion product of steel by carbonic acid.
  • The corrosion scale contains pearlitic remnants from the steel. These remnants are often identified in parts subjected to carbonic acid corrosion.
  • The losses are confined to nozzles, and mostly on the downstream sides of nozzles. Direction changes can induce in acidic breakout from solution, and in rapid losses.

The carbon steel components were replaced with stainless steel to improve performance.

The carbon steel reducer shown was part of a hydrofluoric (HF) acid piping system in a refinery. See Ana Benz et al. “NDT Inspections of HF Acid Lines in Alkylation Units,” Inspectioneering Volume 21, Issue 2, March|April 2015). Routine inspections identified the very thin reducer shown below; it operated in a low acid strength isobutane line from the main fractionator. The pipe thickness was of the order of 0.295 inches. Instead, the reducer thickness was as low as 0.05 inches.

Computed radiography identified the reducer losses as shown on the top left image; next to the welds, the reducer is thin. The top right image shows the reducer’s inside – the pipe to either side of the reducer is not attacked. The bottom left image shows a metallographic cross-section of the attacked reducer next to one of the welds. The bottom right table shows the results of optical emission spectrometry chemical analyses. In this service, the chemical composition of the parts is important, as discussed below.

The table summarizes how the reducer had lower residual elements (RE) than the piping with which it was galvanically coupled. Preferential attack of low RE steel coupled to high RE steel is known to develop in low HF acid strength lines.

The carbon steel components in these piping circuits are under a rigorous periodic inspection program to prevent leaks.

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